This invention relates to a gas-phase process for the regeneration of sulfuric acid, contaminated by organic and other impurities, in which the impurities are oxidized without appreciable decomposition of sulfur trioxide to sulfur dioxide.
Sulfuric acid (H.sub.2 SO.sub.4) is used in numerous industrial chemical reactions where it is not consumed to form the product but becomes contaminated and requires regeneration. For example, sulfuric acid is frequently used as a catalyst, dehydrating agent or carrying agent.
An enormous quantity of sulfuric acid is used by the petroleum industry as an alkylation catalyst. When sulfuric acid is used as an alkylation catalyst, it becomes contaminated primarily with organic impurities and water. Such contaminated sulfuric acid is frequently referred to as "spent sulfuric acid."
Those skilled in the art have long known that spent sulfuric acid could be regenerated by reactions resulting in the dehydrogenation, cracking, thermal decomposition and oxidation of the hydrocarbons, and decomposition of the acid to sulfur dioxide (SO.sub.2). The process is carried out, for example, by injecting large droplets, e.g. up to 1000 microns in diameter, of spent sulfuric acid containing hydrocarbon or organic contaminants into a decomposition furnace heated to 1000.degree. C. or more by combustion of conventional fuels. At furnace conditions vaporization of these large droplets requires several seconds. During this period rapid liquid and gas phase reactions occur between the hydrocarbon and H.sub.2 SO.sub.4. These reactions can be represented by the following reaction sequence: EQU C.sub.n H.sub.m (l)+H.sub.2 SO.sub.4 (l).fwdarw.C.sub.n H.sub.m-2 (l)+SO.sub.2 (g)+2H.sub.2 O(l) EQU C.sub.n H.sub.m (g)+H.sub.2 SO.sub.4 (g).fwdarw.C.sub.n H.sub.m-2 (g)+SO.sub.2 (g)+2H.sub.2 O(g)
Where n=2 to 30 or more and generally n&lt;m&lt;2n+2 Repeated dehydrogenation ultimately yields carbon (soot) and extensive SO.sub.2.
As soon as a droplet of spent acid is vaporized, the sulfuric acid gas begins dissociating to form water and sulfur trioxide according to the reversible reaction: EQU H.sub.2 SO.sub.4 (g)=H.sub.2 O(g)+SO.sub.3 (g)
Gaseous sulfur trioxide rapidly decomposes into sulfur dioxide and oxygen at temperatures approaching 1000.degree. C. by two well-known reactions. At these temperatures the predominant reaction is the reversible thermal decomposition reaction resulting from molecular collision, represented as: EQU SO.sub.3 (g)+M(g)=SO.sub.2 (g)+O(g)+M(g)
where M=N.sub.2, O.sub.2, CO.sub.2, SO.sub.3, etc.
In the second decomposition reaction atomic oxygen reacts with sulfur trioxide, forming the oxygen molecule, and sulfur dioxide. EQU SO.sub.3 (g)+O(g)=SO.sub.2 (g)+O.sub.2 (g)
These two gas-phase reactions, both of which are rapid at temperatures above 850.degree. C., complete the reduction of SO.sub.3 to SO.sub.2 in the furnace.
The SO.sub.2 in the effluent gases is catalytically reoxidized to sulfur trioxide (SO.sub.3) at about 450.degree. C., which is then absorbed by dilute sulfuric acid solution to regenerate fresh concentrated sulfuric acid.
This well-known process is extremely energy consumptive and costly. It utilizes furnace temperatures in excess of 1000.degree. C. at atmospheric pressure to completely decompose SO.sub.3 to SO.sub.2 and to oxidize the hydrocarbon impurities. Added energy and process complexity are required to reoxidize the sulfur dioxide. Additionally, this process results in difficult to control emissions requiring extensive abatement.
Although the process has long been known, it was almost always less costly to generate fresh sulfuric acid directly from elemental sulfur. Spent sulfuric acid or sulfate salts were often simply dumped as chemical wastes prior to enactment of pollution control laws.
Spent sulfuric acid, particularly when derived from sulfuric acid alkylation catalyst, typically contains less than twelve percent impurities including water. Thus, if these impurities can be removed in an economical manner, the acid can be recycled thereby reducing consumption of natural resources and eliminating or reducing the cost of regeneration as currently practiced. The most economical method is to oxidize the hydrocarbon impurities without significantly decomposing the sulfuric acid to sulfur dioxide.
It is therefore an object of this invention to regenerate spent sulfuric acid by a process which minimizes the decomposition of H.sub.2 SO.sub.4 to SO.sub.2.
It is another object of this invention to regenerate spent sulfuric acid by a process which conserves energy and substantially reduces the cost of pollution abatement.
It is another object of this invention to recycle through regeneration sulfuric acid used as an alkylation catalyst.
It is another object of this invention to regenerate and concentrate spent sulfuric acid at sufficiently low cost such that higher acid strength can be maintained in the alkylation units, yielding higher octane alkylates, than is economically feasible using fresh acid.
The method of carrying out these objectives is described herein. Although the examples and discussion are based on regeneration of spent sulfuric acid alkylation catalyst, it should be understood that the process is applicable to other types of spent sulfuric acid containing oxidizable and other minor impurities.